Question

During fixation of one molecule of \(\mathrm{CO}_{2}\) by \(\mathrm{C}_{3}\) plants, number of ATP and \(\mathrm{NADPH}_{2}\) required are (a) 3 ATP and \(2 \mathrm{NADPH}_{2}\) (b) 5 ATP and 2 NADPH \(_{2}\) (c) 12 ATP and \(12 \mathrm{NADPH}_{2}\) (d) \(2 \mathrm{ATP}\) and \(3 \mathrm{NADPH}_{2}\).

Short answer

For the fixation of one molecule of CO2 in C3 plants, 3 ATP and 2 NADPH are required, which corresponds to option (a).

Step-by-step solution

Understand the Process of Photosynthesis in C3 Plants

In C3 plants, the calvin cycle is the pathway in which CO2 is fixed to create sugars. This process requires energy in the form of ATP and reducing power in the form of NADPH. To determine the correct number of ATP and NADPH molecules needed, recall the stoichiometry of the Calvin cycle.

Recall the Stoichiometry of the Calvin Cycle

For the fixation of one molecule of CO2, the Calvin cycle requires 3 molecules of ATP and 2 molecules of NADPH. These energy and reducing power inputs are necessary for the regeneration of ribulose 1,5-bisphosphate (RuBP) and the reduction phase.

Identify the Correct Answer

Given the stoichiometry of the Calvin cycle, the number of ATP and NADPH required for the fixation of one CO2 molecule in C3 plants is 3 ATP and 2 NADPH. Therefore, the correct answer is option (a).

Key concepts

Photosynthesis ATP and NADPH

One of the most critical aspects of photosynthesis in C3 plants, such as beans and rice, is the role that adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH) play. These two molecules serve as energy currency and reducing power, respectively. During the Calvin Cycle—specifically, the conversion of carbon dioxide (CO2) into glucose—ATP provides the energy required for bond formation, while NADPH supplies the electrons required to reduce the fixed carbon into a carbohydrate.

Think of ATP and NADPH as the battery packs powering one of nature's most complex assembly lines—the Calvin Cycle. While the sun's energy is used to initially produce ATP and NADPH during the 'light-dependent reactions' of photosynthesis, it is the 'light-independent reactions,' or the Calvin Cycle, where these molecules are used to convert CO2 into usable sugars for the plant to grow.

CO2 fixation in plants

Carbon dioxide fixation is an essential process whereby plants take in CO2 from the atmosphere and 'fix' it into a stable, organic form that can be used for growth. In C3 plants, this is achieved through the Calvin Cycle, which is aptly named because it fixes carbon in a three-carbon compound, 3-phosphoglycerate (3-PGA). The first step of the cycle involves the enzyme RuBisCO adding CO2 to ribulose 1,5-bisphosphate (RuBP), a five-carbon sugar, resulting in a stable six-carbon compound that immediately splits into two molecules of 3-PGA.

Understanding the intricate steps involved in CO2 fixation can be comparable to following a recipe where precision is key. Each 'ingredient,' or molecule of CO2, requires a specific 'portion' of energy and reducing power—3 ATP and 2 NADPH molecules—to be transformed into a glucose molecule. The exact stoichiometry of the Calvin Cycle is important knowledge for anyone studying the fundamental processes of plant growth and development.

Ribulose 1,5-bisphosphate (RuBP) regeneration

The continuous operation of the Calvin Cycle hinges on the regeneration of ribulose 1,5-bisphosphate (RuBP), which is the molecule that commences the cycle by reacting with CO2. RuBP regeneration is the final phase of the Calvin Cycle and is akin to a loop that must be closed for the cycle to proceed unimpeded. Without RuBP, the plant would not be able to fix carbon and would eventually stop growing.

The plant expends a portion of the energy encapsulated in ATP and the reducing power held by NADPH not only for the initial fixation of CO2 but also to regenerate RuBP. This regeneration process ensures that the cycle is ready to start anew, allowing the plant to continue absorbing and fixing CO2. Each turn of the cycle incorporates one molecule of CO2, so for a single RuBP to be regenerated and available for the next cycle, the Calvin Cycle must turn three times. This reflects the energy and effort nature exerts to maintain the delicate balance of life-supporting processes at the cellular level in plants.